I consider myself pretty good at what other people would call multi-tasking. But when I give the appearance of multi-tasking it's because I'm doing one of two things: either I am rapidly shifting my attention back and forth between two tasks during the spaces in each task when my attention isn't needed there; or I am giving my full attention to only one of the tasks, because the other one is so automatic that I don't have to give it very much attention, like walking.
I haven't done that kind of work for years but just yesterday morning and afternoon, I was keeping an eye on a video transfer I was working on, burning and assembling CD's ( which included assembling the paper case inserts while the CD's were burning, and thinking ahead to which album I'd be burning next and getting things ready for that ), getting boxes and envelopes addressed and ready for mailing, as well as checking email and facebook. I did this because several of the tasks and mini-tasks did not require a lot of attention, and because whenever there was a space in one task chain that required any waiting, I immediately did something to move another task chain a bit further forward. To do this requires a certain amount of mental flexibility and processing speed. If you have it, you have it. If you don't, I suggest finding a job where it's not necessary.